I can see it going 1 of 2 ways, games will be made for the index and then downgraded to run on the quest, or if they sell enough the best games you'll get on the index will be ports over from quest much like we had with consoles at the start.
The £299 price is misleading every review says the cheap head strap is uncomfortable so you will need to spend £50 on the elite, and having thee quest 64Gb isn't much storage so in reality you will want the 256Gb + the Elite which is £448